Fa caldo should always be said with a note of incredulity for maximum effect.

“I tend to exclaim ‘fa caldo!” automatically every time I open the car door on a hot day and get hit in the face

by a wall of hot air, as if I wasn’t fully expecting that to happen. And then I mutter it again as I slide into the hot seat and frantically push at the AC buttons. If the heat is particularly oppressive, it might even become “fa caldissimo!”

You could switch things up a bit, and say che caldo! or che caldo fa! Other useful Italian words for talking (complaining) about Italy’s current weather conditions include canicola and addosso.